1. Field of the Invention
The present invention relates to a resin composition for sealing semiconductor elements or devices (hereinafter simply referred to as "semiconductor(s)"). The resin composition has a low elastic modulus and a low thermal expansion coefficient which does not adversely affect the heat resistance of the semiconductors, and which is excellent in thermal shock resistance and heat resistance during soldering and is therefore useful for sealing electronic parts such as semiconductors where high-reliability is required.
2. Description of the Prior Art
Recently, so-called plastic sealing methods using thermosetting resins represented by epoxy resins have been employed for sealing semiconductors. This is due to economical merits such as low price of the starting materials and ease of mass production. In particular, resins mainly used for sealing are resin compositions principally composed of polyfunctional epoxy resins, novolak type phenol resins and an inorganic filler because of their excellent heat resistance, moldability and electrical properties.
On the other hand, the degree of integration of semiconductor chips has been increased and correspondingly the chips have also been scaled up. Contrary to the scaling up of chips, there has been a tendency for the shape of the chip-containing packages to be made more compact and thinner as in the case of flat packages which is accompanied by changing to high density packaging on a substrate and surface mounting. For this reason, failures have occurred which have not been encountered with the conventional resins for sealing. More specifically, when thermal shock is applied, stress generated in the sealing resin due to the difference in the thermal expansion coefficients between the resin and the chip causes adverse phenomena such as the formation of cracks in the passivating film and cracks in the resin for sealing and a shift of aluminum wiring because of the scaling up of the chips and the reduction in thickness of the resin layer.
Moreover, since the package itself is exposed to a high temperature in a bath of solder during soldering, when surface mounting is performed, the moisture in the package is quickly evaporated. This in turn causes adverse phenomena such as the formation of cracks in the Package, thereby lowering the humidity resistance of the semiconductors and hence reduction in the reliability thereof. Accordingly, it has been desired to develop resins for sealing semiconductors which have reduced stress and excellent heat resistance during soldering even after moisture absorption.
Stress generated in the chip is expressed by the product of the elastic modulus of the resin and the difference in the thermal expansion coefficients between the resin and the chip. One method for reducing such stress is to make the thermal expansion coefficient of the resins small thereby minimizing the difference between the thermal expansion coefficient of the resin and a chip, but the difference is generally large and thus a large amount of inorganic fillers having a low thermal expansion coefficient must be incorporated into the resin to minimize such difference. A large amount of inorganic fillers has already been used practically in such a resin for sealing and, therefore, a further increase in the amount thereof would result in the deterioration of the moldability of the resulting resin composition. On the other hand, it has also been tried to use a plasticizer, a flexible epoxy resin or a phenol resin for the purpose of decreasing the elastic modulus of a resin to thus minimize the stress generated in a resin for sealing during thermal shock, but the hardened product obtained from these methods shows insufficient heat resistance.
Our Japanese Patent Un-examined Publication Nos. Sho 62-270617 and Sho 62-273222 disclose methods for reducing the stress generated in a resin composition during thermal shock while maintaining the desired heat resistance thereof, which comprise uniformly dispersing silicone polymer particles having an average particle size of not more than 1.0 micron in a graft polymer of an epoxy resin and a vinyl polymer. These methods are effective for lowering the stress, but they cannot completely solve the problem of heat resistance during soldering after moisture absorption. More specifically, cracks are formed in a package which absorbs water when it is exposed to an elevated temperature, such as those encountered with a bath of solder, which temperature is higher than the glass transition point of the sealing resin.
The inventors of the present invention have also investigated various methods for eliminating the foregoing problems such as a method for enhancing heat resistance of resins by using a special epoxy resin and a hardener or a method in which a maleimide compound is simultaneously used. These methods are effective for vapour phase soldering employed at a relatively low temperature, but the effects thereof for liquid phase soldering at a relatively high temperature of the order of 260.degree. C. are insufficient.